1. Field of the Invention
The present invention relates to composite nickel fine powder which has characteristic properties suitably used as a material for preparing an internal electrode of laminated ceramic condensers, which is excellent in resistance to oxidation of metal nickel and resistance to diffusion of nickel oxide, in particular, during binder-removal, as well as in thermal shrinkage properties, which can accordingly prevent the occurrence of delamination and the formation of cracks when manufacturing a large-sized laminated ceramic condenser, and which permits the production of a thin, small-sized multilayered ceramic condenser comprising ceramic dielectric layers and internal electrodes, without impairing the dielectric properties and electric properties thereof, and to a method for preparing the composite fine powder.
2. Description of the Prior Art
A laminated ceramic condenser is produced by alternatively putting ceramic dielectric materials and internal electrodes in layers, attaching them by applying pressure and then unifying them by firing. In this respect, an internal electrode for such a laminated ceramic condenser is in general produced by forming metal fine powder as a material for the internal electrode into a paste, printing a layer of the paste on a ceramic substrate, putting a plurality of these printed substrates in layers, thereafter attaching them to one another with pressing and heating to give a unified product and finally heating and firing in a reducing atmosphere. As materials for such an internal electrode, there have been used, for instance, platinum and palladium, but there has recently been developed and advanced a technique in which base metals such as nickel are used in place of precious metals such as platinum and palladium.
When using metallic nickel fine powder as an internal electrode material, however, it has a tendency to undergo rapid thermal shrinkage at a temperature higher than about 400 to 500xc2x0 C., although the tendency depends on the particle size of the powder. For this reason, if fine powder of metal nickel is used as an internal electrode material, a unified product such as that described above is liable to cause delamination and/or crack-formation during firing the same because of the difference between the ceramic substrate and the layer of the metal nickel fine powder in thermal shrinkage characteristics and this has become a serious problem.
To solve these problems, the inventors of this invention have developed a technique for improving the thermal shrinkage characteristics of the metal nickel fine powder by incorporating magnesium and/or calcium into metal nickel fine particle in an amount falling within a specific range and already filed a patent application (Japanese Patent Application Serial No. Hei 9-342793). This technique permits the inhibition of the foregoing rapid thermal shrinkage at a temperature higher than about 400 to 500xc2x0 C., which has been observed for the conventional metal nickel fine powder and can shift the starting temperature of such rapid thermal shrinkage to a higher temperature side in proximity to the range of from about 600 to 700xc2x0 C.
However, the rapid thermal shrinkage-initiating temperature of the internal electrode material should desirably be shifted to the more higher temperature side in order to prevent the occurrence of delamination and/or the formation of cracks during firing. This is because the firing temperature used for preparing a laminated ceramic condenser may vary depending on the ingredients of ceramic dielectric materials, but that for the barium titanate ceramic dielectrics in general ranges from about 1200 to 1400xc2x0 C.
Moreover, when firing is carried out while a ceramic substrate is brought into contact with a metal, the metal is in general oxidized and the resulting metal oxide has a diffusion coefficient higher than that of the ceramic substrate. Accordingly, diffusion easily occurs, at the grain boundaries, from the metal oxide phase having a higher diffusion coefficient to the ceramic phase having a lower diffusion coefficient.
More specifically, when using a paste containing conventional metal nickel fine particles, a part of the metal nickel of the fine particle is oxidized and the resulting nickel oxide diffuses into a ceramic dielectric material. As a result, a part of the internal electrode to be formed has a tendency to disappear, defects are formed in the internal electrode and a part of the ceramic dielectric layer has a tendency to damage its dielectric characteristics due to the formation of ferrite. Accordingly, it has been quite difficult to produce a thin, small-sized laminated ceramic condenser which comprises layers of a ceramic dielectric material and internal electrode layers, without impairing the dielectric characteristics and electric properties thereof.
As has been discussed above in detail, it would be very important that the metal nickel fine powder as a material for preparing a paste used for the preparation of a laminated ceramic condenser must be able to prevent the oxidation of metal nickel of the fine powder and any diffusion of nickel oxide into a ceramic substrate during firing and simultaneously to shift the rapid thermal shrinkage-starting temperature of the metal nickel fine powder to the more higher temperature side so that the thermal shrinkage curve of the nickel fine powder approaches that of the ceramic substrate.
Accordingly, it is an object of the present invention to provide composite nickel fine powder which has characteristic properties suitable for use as a material for preparing an internal electrode of laminated ceramic condensers, which is excellent in resistance to oxidation of metal nickel and resistance to diffusion of nickel oxide, in particular, during the binder-removal stage, which has a thermal shrinkage curve approximately identical to that of the ceramic substrate, which can accordingly prevent the occurrence of delamination and the formation of cracks when manufacturing a large-sized laminated ceramic condenser, and which permits the production of a thin, small-sized multilayered ceramic condenser comprising ceramic dielectric layers and internal electrodes, without impairing the dielectric characteristics and electric properties thereof.
Another object of the present invention is to provide a method for preparing such composite nickel fine powder.
The inventors of this invention have conducted intensive studies to accomplish the foregoing objects, have found that composite nickel fine powder having the foregoing characteristic properties can be obtained by fixing an oxide and/or a complex oxide of a specific metal element to the surface of metal nickel fine particles and that such composite nickel fine powder can be prepared by a wet-carrying process, a dry-carrying process or a semi-dry-carrying process and thus have completed the present invention on the basis of these findings.
According to a first aspect of the present invention, there is provided composite nickel fine powder which is characterized in that at least one member selected from the group consisting of oxides and complex oxides which comprise at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table is carried on the surface of metal nickel fine particles.
According to another aspect of the present invention, there is provided a method for preparing composite nickel fine powder which comprises the steps of adding an aqueous solution containing at least one member selected from the group consisting of water-soluble salts of metal elements each having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table to a slurry containing metal nickel fine particles or metal nickel fine particles whose surface is oxidized; and then adjusting the pH of the mixture with an acid or an alkali to thus carry the metal oxide and/or complex oxide derived from the water-soluble salt on the surface of the metal nickel fine particles.
According to a further aspect of the present invention, there is also provided a method for preparing composite nickel fine powder which comprises the steps of temporally carrying at least one member selected from the group consisting of ultrafine particles of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table on the surface of metal nickel fine particles or metal nickel fine particles whose surface is oxidized and colliding the nickel fine particles temporally carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal nickel fine particles.
According to a still another aspect of the present invention, there is provided a method for preparing composite nickel fine powder which comprises the steps of mixing, with heating, a suspension containing at least one member dispersed therein and selected from the group consisting of ultrafine particles of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table with metal nickel fine particles or metal nickel fine particles whose surface is oxidized; removing the medium of the suspension to thus temporally carry the ultrafine particles on the surface of the metal nickel fine particles; and colliding the nickel fine particles temporally carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal nickel fine particles.
The composite nickel fine powder according to the present invention comprises metal nickel fine particles and at least one member selected from the group consisting of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table, the latter being carried on the surface of the metal nickel fine particles. Therefore, the composite nickel fine powder of the present invention has thermal shrinkage characteristics quite similar to those observed for the ceramic substrate and thus permits prevention of the occurrence of delamination and/or the formation of cracks in the production of a large-sized laminated ceramic condenser. In addition, the composite nickel fine powder has excellent resistance to oxidation and resistance to diffusion during the binder-removal stage and accordingly, permits the production of a thin, small-sized multilayered laminated ceramic condenser which comprises ceramic dielectric materials and internal electrodes, without damaging the dielectric characteristics and electric properties of the condenser.
When producing a thin, small-sized multilayered laminated ceramic condenser which comprises ceramic dielectric materials and internal electrodes, without damaging the dielectric characteristics and electric properties of the condenser, while using a paste containing the composite nickel fine powder of the present invention to thus sufficiently prevent the oxidation of the metal nickel of the fine powder and any diffusion of nickel oxide into the ceramic dielectric layer, observed during the binder-removal stage, or to prevent any damage of the internal electrodes due to disappearance of a part of the internal electrode to be formed and partial loss of the function of the ceramic dielectric layer, the composite nickel fine powder of the present invention preferably comprises metal nickel fine particles and at least one member selected from the group consisting of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 7 or 13 to 14 of Periodic Table and the composite nickel fine powder more preferably comprises metal nickel fine particles and at least one member selected from the group consisting of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2, 3, 4, 7, 13 or 14 of Periodic Table, which is carried on the surface of the metal nickel fine particles.
Furthermore, it is most preferred to use composite nickel fine powder comprising metal nickel fine particles and at least one member selected from the group consisting of oxides of metal elements belonging to Group 2 of Periodic Table, Y, Zr, Al and Si, which is carried on the surface of the metal nickel fine particles.
As the foregoing composite oxides, there may be used in the present invention a variety of composite oxides including the foregoing ones and those listed below.
The composite nickel fine powder of the present invention, when used as a material for the internal electrodes of laminated ceramic condensers, may be those obtained by adhering, to the surface of the metal nickel fine particles, at least one member selected from oxides and complex oxides such as those listed above and at least one member selected from the group consisting of oxides of lanthanum series.
The composite nickel fine powder of the present invention, when used as a material for the internal electrodes of laminated ceramic condensers, preferably comprises metal nickel fine particles and at least one member selected from the group consisting of complex oxides represented by the following general formula:
BamX1xe2x88x92mTinZ1xe2x88x92nO3
(in the formula, X represents Sr, Ca, Mg or Pb; Z represents Zr, Y, Sn or Ge; m is a number ranging from 0 to 1; and n is a number ranging from 0 to 1), the complex oxide being carried on the surface of the metal nickel fine particles. In this respect, the foregoing complex oxides may be used alone or in any combination of at least two of them or the complex oxide may further comprise, as a minor optional component, at least one member selected from the group consisting of, for instance, the foregoing variety of oxides, oxides of lanthanoid elements, bismuth oxide and tantalum oxide.
The foregoing oxides and complex oxides may be, for instance, MgO, CaO, SrO, BaO, ZnO, Al2O3, Ga2O3, Y2O3, SiO2, TiO2, ZrO2, Cr2O3, MnO2, Mn3O4, Nb2O5, BaTiO3, CaTiO3, SrTiO3, BaZrO3, CaZrO3, SrZrO3, (Mg, Ca)TiO3, (Ba, Ca) (Ti, Zr)O3, PbTiO3, Pb(Zr, Ti)O3, (Pb, Ca)TiO3, MgAl2O4 and BaTi4O3, which can be used in combination. Moreover, these oxides and complex oxides may be doped with oxides of, for instance, Nb, W, La, Y and Mo.
The foregoing oxides of lanthanoid elements may be, for instance, Nd2O3, Sm2O3, Dy2O3, Er2O3 and Ho2O3.
The composite nickel fine powder or fine particles of the present invention, when used in the form of a paste for producing an internal electrode of laminated ceramic condensers, preferably have a particle size of not more than 5 xcexcm and more preferably not more than 1 xcexcm.
In addition, the total amount of the foregoing at least one member selected from the group consisting of the foregoing oxides and complex oxides to be carried on the metal nickel fine particle preferably ranges from 0.05 to 10% by weight, more preferably 0.5 to 10% by weight and particularly preferably 1 to 10% by weight on the basis of the total weight of the composite nickel fine powder. This is because if the total amount of the carried oxides and/or complex oxides is less than 0.05% by weight, the effect of the oxides and/or complex oxides is apt to be insufficient, while if it exceeds 10% by weight and such composite nickel fine powder is used as a material for producing an internal electrode of laminated ceramic condensers, these oxide may adversely affect the dielectric characteristics of the resulting condenser.
The metal nickel fine particles or the metal nickel fine particles whose surface is oxidized, which are used in the method of the present invention, may be prepared by either a dry process such as the vapor phase hydrogen-reduction with nickel salt vapor or a wet process wherein metal nickel fine particles are precipitated from an aqueous solution containing a nickel salt under specific conditions through reduction with a reducing agent. When the composite nickel fine powder of the present invention is used in the form of a paste for producing an internal electrode of laminated ceramic condensers, the metal nickel fine particles to be used preferably have a particle size of not more than 5 xcexcm and more preferably not more than 1 xcexcm.
The composite nickel fine powder according to the present invention may be prepared by a wet-carrying process or a dry-carrying process or further a semi-dry-carrying process which comprises the steps of mixing an aqueous suspension of ultrafine particles of a metal oxide or complex oxide with metal nickel fine particles to thus carry the ultrafine particles on the surface of the metal nickel fine particles and then drying the ultrafine particle-carrying metal nickel fine particles.
If the composite nickel fine powder of the present invention is prepared by the wet-carrying process, an aqueous solution containing at least one member selected from the group consisting of water-soluble salts of metal elements each having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table to a slurry containing metal nickel fine particles or metal nickel fine particles whose surface is oxidized, followed by adjusting the pH of the mixture with an acid or an alkali to thus carry the metal oxide and/or complex oxide derived from the water-soluble salt on the surface of the metal nickel fine particles.
If the composite nickel fine powder of the present invention is prepared by the wet-carrying process, the metal nickel fine particles as a raw material may be those prepared by either a dry process or a wet process. In this respect, the composite nickel fine powder may be prepared by easy and integrated wet processing operations at a low cost by preparing metal nickel fine particles as a raw material according to a wet process discussed above, optionally lightly oxidizing the surface of the metal nickel fine particles with an appropriate oxidizing agent through a wet process and then carrying out the method for preparing the composite nickel fine powder according to the wet-carrying process. In this connection, when using the metal nickel fine particles whose surface is lightly oxidized with an appropriate oxidizing agent in advance, the foregoing oxides and/or complex oxides are liable to be easily formed on the surface of the metal nickel fine particles.
The foregoing water-soluble salts used in the composite nickel fine powder-production method of the present invention, which is carried out according to the wet-carrying process, are not restricted to any specific one inasmuch as they are soluble in water and can be converted into water-insoluble oxides or complex oxides. Examples thereof are halides, nitrates, sulfates, oxalates, oxides, and alkali metal salts such as aluminates and silicates, of the foregoing metal elements.
When the composite nickel fine powder-production method of the present invention is carried out by the wet-carrying process, the pH adjusting agent may be selected from either acids or alkalis depending on the kinds of the foregoing water-soluble salts used, but the kinds of these acids and alkalis used are not limited to any specific one. For instance, a sodium hydroxide aqueous solution can be used if the oxides in the parenthesis are formed using the following water-soluble salt:
titanium sulfate (TiO2); manganese sulfate (MnO2);
chromium chloride (Cr2O3); yttrium chloride (Y2O3);
zirconium chloride oxide (ZrO2).
Moreover, dilute sulfuric acid may be used if the oxides in the parenthesis are formed using the following water-soluble salt:
sodium aluminate (Al2O3); sodium silicate (SiO2).
The foregoing water-soluble salts are converted into their oxides or complex oxides and deposited on and carried on the surface of the metal nickel fine particles to give the composite nickel fine powder according to the present invention, by adjusting the pH of the solution of the water-soluble salt as has been discussed above.
If the composite nickel fine powder-production method of the present invention is carried out according to the wet-carrying process, it may further comprise subsequent washing and/or drying steps in addition to the foregoing processing steps. However, the metal oxides and/or complex oxides derived from the foregoing water-soluble salts are carried on the surface of the metal nickel fine particles according to the wet-carrying process, followed by washing and drying and then the metal nickel fine particles on which the oxides and/or complex oxides are carried may be subjected to an additional step such as a treatment with an apparatus such as Angmill (available from HOSOKAWA MICRON CORP. in Japan), Hybridizer (available from Nara Machinery Co., Ltd., in Japan), Mechano-Fusion (available from HOSOKAWA MICRON CORP., in Japan , Coatmizer (available from FREUND INDUSTRIAL Co. Ltd. in Japan), Dispacoat (available from Nisshin Engineering Co., Ltd., in Japan) or Jetmizer (available from Seisin Enterprise Co., Ltd. in Japan), wherein the metal nickel fine particles are collided with each other or with other objects to thus tightly carry the oxides and/or complex oxides on the surface of the metal nickel fine particles.
In the composite nickel fine powder-production method of the present invention carried out according to the dry-carrying process, at least one member selected from the group consisting of ultrafine particles of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table can be temporally carried on the surface of metal nickel fine particles or metal nickel fine particles whose surface is oxidized, followed by colliding the nickel fine particles temporally carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal nickel fine particles.
When the composite nickel fine powder-production method of the present invention is carried out according to the dry-carrying process and the resulting composite nickel fine powder is used in the form of a paste for producing an internal electrode of laminated ceramic condensers, the metal nickel fine particles or the surface-oxidized metal nickel fine particles, which can be used in the method of the invention, preferably have a particle size of not more than 5 xcexcm and more preferably not more than 1 xcexcm. Moreover, the smaller the particle size of the ultrafine particles of these oxides and/or complex oxides, the smaller the amount thereof required for uniformly carrying them on the metal nickel fine particles. For this reason, the particle size thereof is preferably not more than 0.5 xcexcm, more preferably not more than 0.1 xcexcm and most preferably not more than 0.05 xcexcm.
To carry the ultrafine particles of the foregoing metal oxides and/or complex oxides on the surface of the metal nickel fine particles or the metal nickel fine particles whose surface is oxidized, it is necessary to mix the former with the latter and then the metal nickel fine particles temporally carrying the ultrafine particles are collided with each other or other objects to thus ensure the adhesion of the ultrafine particles to the surface of the metal nickel fine particles. Alternatively, the metal nickel fine particles and the ultrafine particles of the metal oxides and/or complex oxides are charged in an apparatus such as Angmill, Hybridizer, Mechano-Fusion, Coatmizer, Dispacoat or Jetmizer to thus simultaneously mix and carry the latter on the former.
In the composite nickel fine powder-production method of the present invention carried out according to the semi-dry-carrying process, a suspension containing at least one member dispersed therein and selected from the group consisting of ultrafine particles of oxides and complex oxides containing at least one metal element having an atomic number ranging from 12 to 56 or 82 and belonging to Group 2 to 14 of Periodic Table is mixed, with heating, with metal nickel fine particles or metal nickel fine particles whose surface is oxidized, followed by removing the medium of the suspension to thus temporally carry the ultrafine particles on the surface of the metal nickel fine particles, and colliding the metal nickel fine particles temporally carrying the ultrafine particles adhered thereto with each other or other objects to thus carry the ultrafine particles on the surface of the metal nickel fine particles.
The metal nickel fine particles or the surface-oxidized metal nickel fine particles which may be used in the foregoing semi-dry-carrying process, as well as the ultrafine particles of the metal oxides and/or complex oxides may be identical to those used in the foregoing dry-carrying process. In addition, the medium in which the ultrafine particles are suspended is not restricted to any particular one and commonly used are, for instance, water, an acidic aqueous solution, a basic aqueous solution, an alcohol or other organic solvents. In this production method, the suspension used may be prepared by dispersing ultrafine particles of a desired oxide or complex oxide in a predetermined concentration of solid contents or may be, for instance, commercially available silica sol, alumina sol, titania sol and barium titanate sol, which may, if necessary, be diluted to adjust the concentration thereof to a desired level.